A styrene-butadiene copolymer produced by an emulsion polymerization method has been known as a rubber used for making a motorcar tire. However, said copolymer has a problem that a motorcar tire comprising said copolymer is not satisfactory from a viewpoint of fuel cost saving, because the copolymer does not have sufficient impact resilience.
In order to produce a rubber having superior impact resilience, JP 60-72907A discloses a production process, which comprises the step of copolymerizing butadiene and styrene in a hydrocarbon solvent using an organolithium compound as an initiator, and a Lewis base such as an ether as a microstructure-controlling agent.
Also, U.S. Pat. No. 5,189,109A discloses a process for producing a modified polymer rubber having improved impact resilience, the process comprising the step of reacting an alkali metal linked to the end of a diene polymer rubber with a specific acrylamide.